Process for the oxidation of alcohols to aldehydes

ABSTRACT

A process for the preparation of an aldehyde which comprises reacting the corresponding alkanol with a solubilized stable free radical nitroxide having the formula: ##STR1## wherein each of R 1 , R 2 , R 3  and R 4  is an alkyl, aryl or heteroatom substituted alkyl group having 1 to about 15 carbon atoms and each of R 5  and R 6  is alkyl, hydrogen, aryl or a substituted heteroatom, nitric acid and a non-basic polar solvent, in the presence of an oxidant, for about eight hours or less at a temperature in the range of from about -10° C. to about 25° C., and thereafter separating out the aldehyde.

FIELD OF THE INVENTION

This invention relates to a process for the preparation for thepreparation of aldehydes by the oxidation of the corresponding alcoholsin the presence of a stable free radical nitroxide, nitric acid, anon-basic polar solvent and an oxidant.

BACKGROUND OF THE INVENTION

It is known to use nitroxyl radicals/oxoammonium salts in the oxidationof primary alcohols to produce aldehydes and acids and secondaryalcohols to ketones (Journal of Organic Chemistry, vol. 52) (12), pp.2559-2562 and Journal of Organic Chemistry, vol. 55, 1990, p. 462-466).

It is reported in the open literature that primary aliphatic alcoholscan be converted to aldehydes in 30-40% yields in presence of catalyticamounts of cuprous chloride, 2,2,6,6,-tetramethylpiperidine-1-oxyl, andatmospheric oxygen (Journal of American Chemical Society, 1984, 106, pp.3374). It is also known that higher yields of aldehydes can be obtainedif stoichiometric amounts of cupric or ferric salts are used instead ofcatalytic amounts of the cuprous salts (Pure and Applied Chemistry, vol.62(2), 1990, pp. 217-222).

OBJECTS OF THE INVENTION

It is an object of this invention to produce aldehydes with highselectivities from alkanols without producing large amounts of otherproducts such as acids and esters.

It has ben found that aldehydes can be produced in high yields and/orwith high selectivities by using catalytic amounts of a stable freeradical nitroxide, nitric acid and an oxidant.

SUMMARY OF THE INVENTION

This invention relates to a process for the preparation of an aldehydewhich comprises reacting the corresponding alkanol with a solubilizedstable free radical nitroxide having the formula: ##STR2## wherein eachof R₁, R₂, R₃ and R₄ is an alkyl, aryl or heteroatom substituted alkylgroup having 1 to about 15 carbon atoms and each of R₅ and R₆ is alky,hydrogen, aryl or a substituted heteroatom, nitric acid, a non-basicpolar solvent and an oxidant, for about eight hours or less at atemperature in the range of about -10° C. to about 25° C., andthereafter separating out the aldehyde.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present process converts alkanols to the corresponding aldehydes bycontacting the alkanol with a solubilized stable free radical nitroixde,nitric acid, a solvent and an oxidant, for about eight hours or less ata temperature in the range of from about 10° C. to about 25° C.

The alkanol reactant suitably comprises one or more alkanols having acarbon number in the range of from about 1 to about 45. An alkanolconsisting essentially of primary, mono-alkanols is preferred. Mostpreferably, the alkanol reactant consists essentially of one or more C₆to C₃₀ primary mon-alkanols. Preference can also be expressed foralkanols having from 8 to about 20 carbon atoms, with C₉ to C₁₈ alkanolsconsidered more preferred about C₁₁ to C₁₈ alkanols considered mostpreferred. As a general rule, the carbon chains of the alkanols may beof either branched or linear (straight-chain) structure, althoughpreference further exists for alkanol reactants in which greater thanabout 50 percent, more preferably greater than about 70 percent and mostpreferably greater than about 90 percent of the molecules are of linear(straight-chain) carbon structure. In large part, such preferencesrelate more to the utility and value of the products than to theoperability or performance of the process of the invention.

The general suitability of such alkanols as reactants in oxidationreactions is well recognized in the art. Examples of specific alkanolsand of commercially available alkanols and alkanol mixtures within thisclass are also well known. Commercially available mixtures of primarymono-alkanols prepared via the oligomerization of ethylene and thehydroformylation or oxidation and hydrolysis of the resulting higherolefins are particularly preferred.

Examples of commercially available alkanol mixtures include the NEODOLalcohols, trademark of and sold by Shell Chemical Company, includingmixtures of C₉, C₁₀ and C₁₁ alkanols (NEODOL 91 Alcohol), mixtures ofC₁₂ and C₁₃ alkanols (NEODOL 23 Alcohol), mixtures of C₁₂, C₁₃, C₁₄, andC₁₅ aldanols (NEODOL 25 Alcohol), and mixtures of C₁₄ and C₁₅ alkanols(NEODOL 45 Alcohol); the ALFOL Alcohols, trademark of and sold by VistaChemical Company, including mixtures of C₁₀ and C₁₂ alkanols (ALFOL1012), mixtures of C₁₂ and C₁₄ alkanols (ALFOL 1214), mixtures of C₁₆and C₁₈ alkanols (ALFOL 1618), and mixtures of C₁₆, C₁₈ and C₂₀ alkanols(ALFOL 1620); the EPAL Alcohols, trademark of and sold by Ethyl ChemicalCompany, including mixtures of C₁₀ and C₁₂ alkanols (EPAL 1012),mixtures of C₁₂ and C₁₄ alkanols (EPAL 1214), and mixtures of C₁₄, C₁₆,and C₁₈ alkanols (EPAL 1418); and the TERGITOL-L Alcohols, trademark ofand sold by Union Carbide Corporation, including mixtures of C₁₂, C₁₃,C₁₄, and C₁₅ alkanols (TERGITOL-L 125). Also very suitable are thecommercially available alkanols prepared by the reduction of naturallyoccurring fatty esters, for example, the CO and TA products of Proctorand Gamble Company and the TA alchohols of Ashland Oil Company.

The term "stable free radical nitroxide" as used herein shall mean afree radical nitroxide that can be prepared by conventional chemicalreaction or examined in a static system by normal methods ofspectroscopy. Generally, the stable free radical nitroxides of thepresent invention have a half life of at least one year. The term"stable free radical" shall also be understood to include the precursorto a stable free radical from which the stable free radical may beproduced in situ.

The stable free radical nitroxides, as used in the present process, areprecursors to catalysts, i.e., oxoammonium salts, active for theoxidation of alkanols to the corresponding aldehydes. These catalystsare generated in situ by the oxidation of a stable free radicalnitroxide to an oxoammonium salt with an oxygen-containing oxidant. Thestable free radical nitroxide can be obtained by the oxidation ofsecondary amines or hydroxylamines.

The stable free radical nitroxides which are suitable for use in theinstant invention have the formula: ##STR3## wherein each of R₁, R₂, R₃and R₄ is an alkyl, aryl or heteroatom substituted alkyl group having 1to about 15 carbon atoms and no hydrogen is bound to the remainingvalences on the carbon atoms bound to the nitrogen, and each of R₅ andR₆ is alky, hydrogen, aryl or a substituted hexeroatom. As used herein,the term "alkyl" is meant to include cycloalkyl. The alkyl (orheteroatom substituted) groups R₁ -R₄ may be the same or different, andpreferably contain 1 to 15 carbon atoms. Preferably, R₁ -R₄ are methyl,ethyl, or propyl groups. In addition to hydrogen, the heteroatomsubstituents may include, halogen, oxygen, nitrogen and the like.Preferably, one of R₅ and R₆ is hydrogen while the other is a heteroatomwhich does not interfere with the reaction. Suitable substitutedheteroatoms include --OR, ##STR4## O--SO₃ H, --O-- polymer and the like.

In a preferred embodiment, the nitroxide is selected from the groupconsisting of 2,2,6,6- tetramethyl-piperidine-1-oxyl, 4-hydroxy-2,2,6,6-tetramethyl-piperidine-1-oxyl,4-oxo-2,2,6,6-tetramethyl-piperidient-1-oxyl, 2,2,6,6-tetramethyl-piperidine-1-oxyl-4-sulfate,4-alkoxy-2,2,6,6-tetramethyl-piperidine-1oxyl and mixtures thereof, with2,2,6,6-tetramethyl-piperidine-1-oxyl,2,2,6,6-tetramethyl-piperidine1-oxyl-4-sulfate, and4-alkoxy-2,2,6,6-tetramethyl-piperidine-1-oxyl being particularlypreferred.

As used herein, the term "nitric acid" refers to nitric acid, fumingnitric acid or nitrous acid generated by contacting alkali metal nitritewith mineral acid. The nitric acid suitable for use in the presentinvention typically has a concentration in the range of from about 50percent to about 100 percent, preferably about 70 percent. Generally, anamount of nitric acid in the range of from about 5 mole percent to about100 mole percent, basis the number of moles of starting alkanol is used.If excess nitric acid is used and the reaction mixture becomes tooacidic, the reaction stops. The nitric acid is typically added to thereaction mixture after all of the other reactants have been added. Whilenot wishing to be bound by any particular theory, it is believed thatnitrogen oxides (NO_(x)) are generated in the reaction and are theactive species in the reaction.

The oxidants suitable for use in the instant invention are thosecompounds which are capable of oxidizing the stable free radicalnitroxide to the oxoammonium salt. Suitable oxidants include oxygen oran oxygen-containing gas such as air. Whereas pure oxygen is preferredto accomplish the desired conversion, the oxygen can be diluted with aninert gas such as nitrogen, helium, argon, or other similar gas is alsosuitable. While air can be used as the oxidant, the reaction rate ismuch slower. For purposes of increasing the reaction rate, higher O₂pressures such as, for example, 1000 psi can be utilized. In a preferredembodiment, pure oxygen is used as the oxidant and it is bubbled intothe reaction solution. In another embodiment, air can be bubbled throughthe reaction solution.

The reaction in the present invention is carried out utilizing asolubilized free radical nitroxide, The solvent is generally a non-basicpolar solvent (i.e., a solvent which does not form a strong complex withthe oxoammonium ion) which is to a large extent miscible in water and inwhich the alkanol is readily soluble. While non polar solvents affordhigh conversions under comparable conditions of temperature, time andpressure, the use of non polar solvents results in undesirably highamounts of aldehyde dimerization products. Basic polar solvents have atendency to shut down the reaction, thus resulting in incompleteconversions. Polar solvents which are most suitable are those which areinert in the reaction. The solvent may be added to the reaction mixture,or alternatively, the nitroxide may be dissolved in the solvent prior toaddition of the nitroxide to the reaction medium. The solvent istypically selected from the group consisting of acetonitrile, sulfolane,tertiary alcohols such as tertiary butyl alcohol, and mixtures thereof.In a preferred embodiment, the solvent is acetonitrile. The amount ofsolvent utilized in the process is typically in the range of from about20:1 to about 0.5:1, preferably from about 10:1 to about 5:1, basis theweight of the starting alkanol.

The amounts and concentrations of the reactants utilized in the processof the instant invention can vary within wide ranges. The amount ofstable free radical nitroxide is typically in the range of from about 1mole percent by weight to about 50 mole percent, preferably from about 5mole percent by weight to about 20 percent, basis the number of moles ofstarting alkanol. Generally, the amount of nitric acid is in the rangeof from about 5 mole percent to about 100 mole percent, preferably fromabout 25 mole percent to about 50 mole percent, basis the number ofmoles of the starting alkanol.

The process of the present invention is typically conducted under mildconditions, with good results being obtained using a temperature in therange of from about -10° C. to about 25° C., and preferably in the rangeof from about 10° C. to about 20° C. Reaction pressures are not criticalalthough higher pressures result in increased reaction rates. Pressuresin the range of from about atmospheric pressure up to about 1000 psigcan be employed with good results. The time of reaction required inorder to obtain a high selectivity to aldehydes is typically about eighthours or less. The time in which the reaction proceeds to aldehydes may,however, be longer in the event that basic solvents such as acetone,dimethylformanide, and N-methyl pyrrolidone are used. The optimum timesof maximizing the selectivity to aldehydes using a particular solventcan be readily determined by one skilled in the art with a minimalamount of routine experimentation.

The process of the instant invention can be carried out either batchwiseor continuously, using a stirrer equipped reactor or other well knowncontacting technique to achieve adequate mixing. Preferred reactionconditions, e.g., temperature, pressure, flow rates, etc., vary somewhatdepending on the specific nitroxide utilized and on the concentration ofthe nitroxide.

The process of the instant invention can be carried out in a variety ofways. For example, 0.032 moles of alkanol, 0.006 moles of the nitroxide,and solvent may be added to the reaction vessel, followed by theaddition of 0.016 moles of 70 percent nitric acid and bubbling O₂through the reaction mixture. Following the reaction, the product may beseparated from the reaction mixture using conventional procedures suchas extraction using a suitable extraction solvent such as, for example,ethyl acetate; evaporation wherein the solvent is stripped from thereaction mixture by using heat or vacuum. The reaction product can bepurified by a number of conventional means such as, for example,distillation.

Depending upon process conditions and the nitroxide used, theselectivity to aldehydes obtained by this invention can be greater thanabout 95% of starting alkanol material being converted. The productsproduced by the instant process can be used in a variety ofapplications. For example, these products can be used as intermediatesin the preparation of esters, imides, imines, amines and acids.

The ranges and limitations provided in the instant specification andclaims are those which are believed to particularly point out anddistinctly claim the present invention. It is, however, understood thatother ranges and limitations which perform substantially the samefunction in the same or substantially the same manner to obtain the sameor substantially the same result are intended to be within the scope ofthe instant invention as defined by the instant specification andclaims.

The process of this invention will be further described by the followingembodiments which are provided for illustration and are not to beconstrued as limiting the invention.

ILLUSTRATIVE EMBODIMENTS Example 1

6 Grams of 1-dodecanol, 1 gram of 2,2,6,6-tetramethylpiperidine-1-oxyl,25 milliliters of acetonitrile and 1 gram of 70% nitric acid werecharged to a 100 milliliter round bottomed flask. An O₂ stream was thenbubbled through the mixture. The reaction temperature was held at 10° C.over an 8 hour period. The results are presented in Table I.

EXAMPLE 2

6 Grams of 1-dodecanol, 1 gram of 2,2,6,6-tetramethylpiperidine-1-oxyl,25 milliliters of tertiary butyl alcohol, 1 gram of 70% nitric acid and2 grams of water were charged to a 100 milliliter round bottomed flask.A stream of air was then bubbled through the mixture. The reactiontemperature was held at 0°C. over a 3 hour period. The results arepresented in Table I.

Example 3

6 Grams of 1-dodecanol, 1 gram of 2,2,6,6- tetramethylpiperidine-1-oxyl, 25 milliliters of glyme, and 1 gram of 70% nitric acid werecharged to a 100 milliliter round bottomed flask. An O₂ stream was thenbubbled through the mixture. The reaction temperature was held at 10° C.over a 6 hour period. The results are presented in Table I.

Comparative Example A

Comparative Example A was carried out in a manner similar to Example 1except that the temperature was held at 35° C. for six hours. Theresults are presented in Table I.

Comparative Example B

Comparative Example B was carried out in a manner similar to Example 1except that the reaction temperature wall allowed to warm to 23° C. overan additional 16 hour period. The results are presented in Table I.

As can be seen in Table I, temperatures exceeding 25° C or highlyextended reaction times, i.e., times greater than about 8 hours, affordcarboxylic acids rather than the desired aldehydes.

                  TABLE I                                                         ______________________________________                                        Oxidation Of Alkanols to Aldehydes                                                                       % Sel.                                                              % Sel.    Esters +  % Sel.                                          % Conversion                                                                            Aldehydes Heavies   Acids                                    ______________________________________                                        Example 1                                                                              72            >99.9     <0.1   0                                     Example 2                                                                              24          >99       <1       0                                     Example 3                                                                              34          >99       <1      <1                                     Comparative                                                                            >99.9       33        3       64                                     Example A                                                                     Comparative                                                                            96          22        2       76                                     Example B                                                                     ______________________________________                                    

What is claimed is:
 1. A process for the preparation of an aldehydewhich comprises reacting the corresponding alkanol having a carbonnumber in the range of from about 1 to about 45 with a solubilizedstable free radical nitroxide having the formula: ##STR5## wherein eachof R₁, R₂, R₃ and R₄ is an alkyl, aryl or heteroatom substituted alkylgroup having 1 to about 15 carbon atoms and each of R₅ and R₆ is alky,hydrogen, aryl or a substituted heteroatom, nitric acid and an non-basicpolar solvent selected from the group consisting of acetonitrile,sulfolane, tertiary butyl alcohol, tertiaryl amyl alcohol, and mixturesthereof, in the presence of an oxygen-containing gas, for about eighthours or less at a temperature in the range of from about -10° C. toabout 25° C., and thereafter separating out the aldehyde.
 2. The processof claim 1 wherein the solubilized stable free radical nitroxide isselected from the group consisting of2,2,6,6-tetramethyl-piperidine-1-oxyl, 4-hydroxy-2,2,6,6-tetramethyl-piperidine-1-oxyl,4-oxo-2,2,6,6-tetramethyl-piperidine-1-oxyl, 2,2,6,6-tetramethyl-piperidine-1-oxyl-4-sulfate,4-alkoxy-2,26,6-tetramethyl-piperidine-1-oxyl and mixtures thereof. 3.The process of claim 2 wherein the solubilized stable free radicalnitroxide is selected from the group consisting of 2,2,6,6-tetramethyl-piperidine-1-oxyl,2,2,6,6-tetramethyl-piperidine-1-oxyl-4-sulfate,4-alkoxy-2,2,6,6-tetramethyl-piperidine-1-oxyl and mixtures thereof. 4.The process of claim 1 wherein said solvent is acetonitrile.
 5. Theprocess of claim 1 wherein said is nitric acid has a concentration inthe range of from about 50 percent to about 100 percent.
 6. The processof claim 5 wherein said nitric acid has a concentration of about 70percent.
 7. The process of claim 1 wherein said the amount of nitricacid utilized is in the range of from about 5 mole percent to about 100mole percent.
 8. The process of claim 1 wherein said alkanol iscontacted with said solubilized stable free radical nitroxide, followedby the addition thereto of said nitric acid and said oxidant.
 9. Theprocess of claim 8 wherein the amount of solubilized stable free radicalnitroxide is in the range of from about 1 mole percent to about 50 molepercent, basis the number of moles of said alkanol.
 10. The process ofclaim 8 wherein the amount of solubilized stable free radical nitroxideis in the range of from 5 mole percent to about 20 mole percent, basisthe number of moles of said alkanol.
 11. The process of claim 8 whereinsaid the amount of nitric acid utilized is in the range of from about 5mole percent to about 100 mole percent.
 12. The process of claim 1wherein said oxygen-containing gas is selected from pure oxygen andoxygen in air.
 13. The process of claim 12 wherein saidoxygen-containing gas is pure oxygen.
 14. The process of claim 1 whereinsaid process is carried out at a temperature in the range of from about10° C. to about 20° C. and at atmospheric pressure.